The present invention relates generally to Uninterruptable Power Supply (UPS) and more specifically to a high-efficiency UPS which provides a smooth transition from utility power to battery power without dropout.
Uninterruptable Power Supplies are used in many electrical and electronic systems to protect against loss or degradation of operation in the event of a utility power outage. A typical UPS system will provide backup power for the protected system from a battery source that is kept charged from the utility. Management of battery charging is often pad of UPS system operation.
UPS systems are commonly referred to as "on-line" or "off-line". An on-line UPS will keep certain electronics circuitry in a powered condition for the purpose of more quickly detecting the presence of a power outage and apply backup power to the protected system. An off-line UPS supplies power to a minimum of electronics circuitry, and in the event of utility power outage other electronic circuitry must be powered up so that the process of transferring to backup power can be implemented.
An example of a prior art on-line UPS system 10 is shown in FIG. 1. Utility Power 11, which in this case is 120 VAC, is applied to Bridge Rectifier 12. Bridge Rectifier 12 rectifies AC into DC (direct current) which is applied to the input of Half-bridge Converter 14. The purpose of Half-bridge Converter 14 is to provide a regulated DC output 16 which, in this example, is 36 volts DC. Output 16 is used for battery charging and for operation of Double Forward Converter 18. Battery charging is supervised by the Battery Monitoring and Charge Control 34, which regulates the output of the Half-bridge Converter 14 to provide the proper charging voltage and current to the 36 volt Battery 38. Battery Monitoring and Charge Control 34 also provides an error signal through Half-bridge Pulse Width Modulator (PWM) 36 which is applied to the control input of Half-bridge Converter 14 and is part of the feedback loop which regulates output 16. Double Forward Converter 18 is used to provide a regulated 60 VDC Output 20. 60 VDC Output 20 is applied to the input of Fullwave (F/W) Bridge Inverter 22, which performs the function of converting the 60 VDC output 20 to a 60 VAC output 24 using Control Signal 30. Control Signal 30 is provided by the F/W Bridge Inverter Controller portion of Forward and F/W Bridge Controller 32. Pulse Width control signal 26 is generated by the Forward Converter Controller portion of Forward and F/W Bridge Controller 32. The input signal 28 to Forward and F/W Bridge Controller 32 contains voltage and/or current information, which is derived from sensors which monitor 60 VDC Output 20. Signal 28 furnishes voltage and load information from which Forward and F/W Bridge Controller 32 generates Pulse Width control signal 26, part of the feedback loop which regulates 60 VDC Output 20. Forward and F/W Bridge Controller 32 also generates control signal 30 which controls operation of F/W Bridge Inverter 22.
The on-line UPS will respond quickly to utility outage occurrences. This can be seen by noting that the Battery 38 is used directly in the power chain, from 36 VDC at Output 16 to 60 VDC at output 20 to the final 60 VAC Output 24. In the event of loss of Utility Power 42, there is nothing to switch--the battery 38 is already "on-line" which means that backup power is immediately available and applied to the protected system. The power efficiency of on-line UPS systems is reduced because two converters, Half-bridge Converter 14 and Double Forward Converter 18, are utilized instead of a single converter which could be more efficient. Most on-line UPS systems use a single inverter operating at the utility AC frequency instead of Double Forward Converter 18 and F/W Bridge Inverter 22, but this results in increased size and weight because low frequency power transformers and wave shaping inductors are required to provide a sine or quasi-square wave output. Note also that two converters generate more complex electromagnetic interference (EMI) characteristics that a single converter.
Referring to FIG. 2, a typical off-line non-switching UPS system 40 is shown. When there is a power outage, the Ferroresonant Transformer 46 is powered by a Regulated DC-AC Inverter 52 operating from the system's standby battery 48. Regulated DC-AC Inverter 52 is normally in a non-operating state until Transfer Controller 54 detects a problem in the AC output 56 at which time Regulated DC-AC Inverter 52 is started and power is applied to Ferroresonant Transformer 46. The major problem with this approach is time lapses, or "drop-outs", between loss of AC mains, sensing that loss, and getting the Regulated DC-AC Inverter 52 in operation. In most UPS systems now in use this results in a momentary loss of output power.
The operation of the off-line non-switching UPS system 40 is as follows. The power available from the Utility Power 42 (120 VAC) is passed through Electronic Switch 44 to the Ferroresonant Transformer 46 which generates a quasi-square waveform, AC output 56. When Utility Power 42 is lost, Transfer Controller 54 senses the drop in AC output 56 and switches off Electronic Switch 44 while switching on Electronic Switch 50 and Battery 48 to Regulated DC-AC Inverter 52. Time is lost in making this transition and in sensing the actual droop in AC output 56 since the output waveshape is highly dependent upon the degree of load current being passed through Ferroresonant Transformer 46. Therefore, the sensing circuits in Transfer Controller 54 will introduce some delay in order to "make sure" that there really is a loss. More sophisticated controllers may also sense Utility Power 42 to bypass the delays in the response of Ferroresonant Transformer 46, but still there is a finite time required to get Regulated DC-AC Inverter 52 on-line from a cold start.
When Utility Power 42 is again available, Transfer Controller 54 waits for some time to retransfer back to Ferroresonant Transformer 46 in order to protect against nuisance transfers, such as momentary power restorations, spikes, and so forth. This time may be as much as one or several minutes. When the transfer is made, however, there is usually a cycle or two of AC lost until Ferroresonant Transformer 46 stabilizes.
Existing UPS design approaches for cable television (CATV) systems typically fall into one of two categories: a) ferroresonant power supplies, and b) switch mode power supplies. Of the two types, the ferroresonant variety is by far the most prevalent due to simplicity of construction but in general has the inability to maintain output when switching from utility power to battery, causing a drop in output which cannot be tolerated in the new generation CATV systems now being developed. The ferroresonant transformer reduces the 120 VAC utility power to the 60 VAC needed for CATV applications, is capable by design of regulating the output voltage versus input line voltage variations and output load current changes, and is simple and cost-effective but suffers from lack of efficiency at all load currents other than its design maximum current. Conventional UPS CATV systems rely upon sensing the loss of utility power and switching to a battery powered inverter to generate the AC output power. A full cycle of AC power (17 mS at 60 Hz) or more can elapse before the inverter comes on-line, during which time power to the CATV system is lost. This loss may be tolerable if only television transmissions are being handled by the cable, but becomes intolerable if digital data is also being transmitted.
The switch mode supply, which does not have this problem, is inefficient in its handling of power which results in higher costs for utility power and a high degree of
The switch mode supply, which does not have this problem, is inefficient in its handling of power which results in higher costs for utility power and a high degree of heat production. Power is always being processed by the DC-DC converters even when utility power is present, hence the poor efficiency. There is, however, no switching of the battery converter to bring the battery on-line, and therefore no dropout period. The on-line system is not commonly found in the CATV field due to its poor efficiency.
Off-line and on-line UPS systems both possess advantages which make them attractive for different reasons. The conventional on-line UPS system ensures uninterrupted operation for the protected system while the conventional off-line UPS system offers the advantage of better operating efficiency than the on-line UPS system. There are, however, also disadvantages of both the on-line UPS system and the off-line UPS system. The conventional on-line UPS system is characterized by poor operational efficiency since two, rather than one, converters must be utilized to guarantee continuous operation of the protected system. This required converter circuitry also translates into increased size, weight and EMI concerns for the on-line UPS system. The typical off-line UPS system is characterized by time lapses and spikes when it becomes necessary to use the standby battery rather than utility power to supply power to the protected system. At the current time, a multiplicity of design approaches are necessary in order to compensate for these and other disadvantages of conventional on-line and off-line UPS systems. There is, therefore, an unmet need in the art to have a UPS system that capitalizes on the advantages of conventional on-line and off-line UPS systems while minimizing their disadvantages.